1. Field of the Invention
This invention relates to an ignition timing control system for internal combustion engines, and more particularly to an ignition timing control system for an internal combustion engine having a system for changing the valve timing of intake valves and/or exhaust valves thereof.
2. Prior Art
Amongst conventional methods for electronically controlling the ignition timing of an internal combustion engine, there is widely known a method of controlling the ignition timing by the use of a spark ignition advance value set in dependence on operating conditions of the engine.
Conventional ignition timing control systems employing this method include an ignition timing control system which delimits a retard-side limit of a spark advance value corrected toward a retarded side in accordance with engine operating parameters etc., by means of a retard limiting value set in dependence on engine rotational speed to thereby prevent a misfire or a flashover due to sudden acceleration or sudden deceleration of the engine, as disclosed e.g. in Japanese Provisional Patent Publication (Kokai) No. 62-17666.
Recently, internal combustion engines have been proposed, which have a valve timing changing system for changing the valve opening period and/or lift (hereinafter generically called "valve timing") of intake valves and/or exhaust valves of the engine in dependence on the engine rotational speed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 2-102347.
Such a valve timing changing system is adapted to select at least high-speed valve timing suitable for engine operation in a high engine rotational speed region, and low-speed valve timing suitable for engine operation in a low-engine rotational speed region so as to enhance the trapping efficiency (combustion efficiency) of the engine and hence increase the engine output. Further, the value timing changing system is also capable of controlling the valve timing in response to operating parameters of the engine such as engine rotational speed and engine load, to thereby enable to always optimize the combustion efficiency.
In an internal combustion engine equipped with a valve timing changing system, the charging efficiency .eta. of a mixture drawn into the engine cylinders differs between when high-speed valve timing is selected and when low-speed valve timing is selected, even if other engine operating parameters such as intake air temperature remain unchanged. More specifically, as shown in FIG. 5, if low-speed valve timing (LoV/T) is selected when the engine rotational speed NE is higher than a predetermined rotational speed NE1, the charging efficiency .eta. is lower than if high-speed valve timing (HiV/T) is selected, so that the mixture is drawn into the engine cylinders in smaller quantities. If a smaller amount of fuel, which is conductive, contained in the mixture is thus present within the engine cylinder, the voltage required to be applied to the spark plug to cause sparking thereof (hereinafter called "spark voltage") becomes higher, which can result in a misfire or a flashover. To avoid this, it is necessary to change the retard-side limit of ignition timing in response to changeover of the valve timing.
Therefore, if a retard limiting value defining the retard-side limit of ignition timing as disclosed in Japanese Patent Publication No. 62-17666, hereinabove referred to, which value is set to a constant value suitable for an internal combustion engine having uncontrolled valve timing, is applied to control the ignition timing of an internal combustion engine having controlled valve timing, there is a problem of occurrence of a misfire or a flashover due to elevated spark voltage of the spark plug encountered when low-speed valve timing is selected when the engine rotational speed is in a high rotational speed region for selecting high-speed valve timing.